We use SAR data from the European ERS-1 and ERS-2 satellites acquired
between May 1992 and January 2001 to construct interferograms across
the CSAF. Processing was carried out using the Caltech/JPL software
ROI_PAC (Rosen et al., 2004), with a bridge unwrapping
algorithm. Topographic effects were removed using a 90 m digital
elevation model derived from Shuttle Radar Topography Mission (SRTM)
data (Farr and Kobrick, 2000; Rodriguez et al.,
2006). Agriculture in the Salinas valley and the San Joaquin basin
results in temporal decorrelation in many of the interferograms, and
steep topography, particularly on the northeast side of the fault,
leads to geometrical decorrelation. Collectively, these zones of
incoherence lead to isolated patches in the unwrapped
interferograms. Figure
2.42 shows a stack of twelve interferograms which can
be unwrapped consistently across the fault. The stacking process
increases the signal-to-noise ratio above that of individual
interferograms, so that a more robust identification of spatial
variations in creep rate can be made. The stack assumes a linear
velocity for each pixel. The fault is clearly delineated by the abrupt
offset running northwest to southeast across the stack image. The
displacement gradient near the fault is much greater than would be
expected for a fault locked to the bottom of the seismogenic layer
(about 12-15 km in this region), implying that significant shallow
slip occurred during the decade of observation. The positive range
change on the southwest part of the image is enhanced in the Salinas
Valley. We surmise that this enhancement is due to subsidence caused
by aquifer discharge in this highly agricultual area. The area of
10 mm/year positive range change in the southeast quadrant of the
stack exactly coincides with the town of Coalinga and the nearby oil
fields. It is possible that this range change anomaly is due to
pumping of oil. The agreement of our 9-year creep rate with estimates
obtained by other workers over earlier and/or longer periods of time
show that creep rate on the years to decadal time scale has been
approximately constant over the last 30 years. If there was any
increase in creep rate as a result of either the 1857 Fort Tejon or
the 1906 San Francisco earthquake, then presumably the rate has now
levelled off.

Figure 2.42:
Stack of 12 descending ERS interferograms made from SAR scenes
acquired between May 1992 and January 2001. Colours show range change,
which is the surface displacement in the line of sight between the
satellite and the ground. Red is positive range change, i.e. motion
away from the satelite. Arrows show satellite ground track (towards
south-southwest) and look direction (towards west-northwest). Fault
traces for the CSAF and Calaveras Fault are marked. The line-of-sight
offset rate across the fault is 10 mm/year, which is equivalent to a
right lateral displacement rate of 32 mm/year. Grey ellipse in bottom
right outlines area of suspected subsidence, possibly related to oil
pumping near Coalinga. Subsidence may also be occurring in the
agricultural Salinas Valley.